In reading or writing information, such as data, on a disk, it is important to maintain the transducer or read/write head, which is used to read or write the information, in alignment relative to the center of the continuous track on which information is stored or to be stored. This alignment is important so that the information can be read accurately or stored properly. A problem commonly arises however in that the axis of rotation of the disk is not precisely the same as the center of the concentric tracks located on the disk. This lack of correspondence between the axis of rotation of the disk and the center of the concentric disk tracks results in a displacement of the head relative to the track center during the spinning or rotating of the disk, i.e., the head is not maintained relative to the center of the track during rotation of the disk.
This displacement from the track center may be due to a number of factors including, for example, the disk was not balanced properly so that the track center is offset relative to the disk axis of rotation. Alternatively, when the disk system was assembled initially, the axis of disk rotation was not properly aligned with the center of the concentric tracks.
The distance from the track center can be characterized as an error signal created by holding the head stationary relative to the center of one disk track and permitting the disk to rotate relative to the head. The error signal comprises the distance the head is away from the center of the one track at each instance in time during one revolution of the disk. In the case in which the head stays in alignment relative to the center of the track, a straight line is produced, i.e., there is no error in the error signal.
In a closed loop servo system involving the movement of the head relative to a concentric track, this displacement can be compensated for to a minimum degree. However, as the distance becomes relatively large, the closed loop servo system is unable to generate sufficient compensation or correction.
The present invention provides a solution to the aforementioned problem associated with disk runout. Basically, the error signal can be defined in terms of a sinusoidal signal having a number of harmonics plus a DC component or offset from a known, reference level. Because the error signal is primarily made up of the sinusoid defined by the fundamental frequency, the runout can be defined to a very good approximation as including only the offset and the fundamental frequency component of the sinusoid. The present invention uses this representation of the runout signal to generate a signal substantially equal to that of the determined runout. This generated signal is applied to cause the head to move at the desired times so that it is maintained in alignment relative to the center of the disk track having the information to be read or on which the information is to be written.